The present invention relates to a digital broadcast receiving apparatus and to a digital broadcast receiving method that are applicable to hierarchical transmission digital broadcasting.
Recently, with the commencement of CS (Communications Satellite) digital broadcasting, followed by BS (Broadcasting Satellite) digital broadcasting, satellite digital broadcasting is becoming increasing popular. In digital broadcasting, one wave can carry a plurality of image, sound and other data contents, and data can be compressed without quality deterioration, making it possible to set a number of channels to a transmission bandwidth, that is typically assigned to one channel in conventional analog broadcasting.
Further, terrestrial digital broadcasting is scheduled to start in 2003. According to the proposed plan, terrestrial broadcasting will employ a hierarchical transmission method in which the transmission bandwidth assigned to each broadcasting station is divided into a number of segments, and a plurality of layers, each consisting of one or more segments, will be transmitted concurrently. In such hierarchical transmission, maximizing the number of segments constituting a layer provides for higher quality data transmission, but results in less robust mobile reception by receivers on movable bodies, such as automobiles. Therefore, each broadcasting station intends to use a plurality of layers that vary in the number of segments therein, so as to provide a plurality of services suited respectively for fixed reception by home receivers, etc., mobile reception by automobile receivers, etc. and/or portable reception by portable telephones. In response to this approach, electrical equipment manufacturers and others are developing digital broadcast reception apparatuses that are applicable to hierarchical transmission terrestrial digital broadcast.
When a digital broadcast signal, either from a radio wave tower or a relay station of a broadcasting station, is received by a digital broadcast receiving apparatus mounted on a movable body, such as an automobile, the field strength of the received digital broadcast signal changes, since the receiving apparatus is sometimes shaded from the wave by a building or the like and the distance from the station changes as the movable body travels. In addition, while the movable body is getting closer to or farther from the station, each frequency of the carrier wave received directly from the station differs from that of a reflected wave received via a building due to the Doppler effect. Because of such fading (frequency interference), this changes the field strength of the received digital broadcast signal and makes it difficult for the digital broadcast receiving apparatus to appropriately choose from carrier waves, each consisting of multiple carrier frequencies modulated so as to encode data, resulting in a deteriorated image and poor sound quality. Further, the field strength of the received digital broadcast signal may also weaken when the movable body is exposed to bad weather conditions and is moving at a high speed.
Therefore, the image and sound reproduced by a digital broadcast receiving apparatus on a movable body is interrupted or confused as the case may be. If the image and sound is degraded in like this way, the user of the digital broadcast receiving apparatus tends to feel uncomfortable and is motivated to operate the apparatus to select another channel or another layer of the same channel. Further, if the image and sound are interrupted, commercials cannot be provided to the user, resulting in a lowered advertising potential.